1. Field of the Invention
This invention relates to a method for dynamic analysis of vibration of machinery, and in particular, to a method for measuring vibration in a machine. Measurement of the vibration allows analysis of the causes of vibration and the analytical determination of modifications to minimise vibration.
In machinery having a rotating member, for example having a shaft mounted for axial rotation by means of bearings and driven in rotation within a casing, excessive vibration is undesirable and may lead to failure of the machine.
2. Description of the Prior Art
Hitherto, analysis of vibration of machinery has been conducted by measuring vibration displacement using special transducers to obtain a signal which varies in time with respect to displacement of a member. Usually a number of transducers are used to obtain displacement measurement in three mutually perpendicular directions. In the case of a machine having a rotating shaft disposed within a casing, the absolute displacement of the shaft is obtained by having transducers mounted to the machine casing to measure relative displacement of the shaft with respect to the casing and having a further series of transducers mounted to the casing to measure the displacement of the casing with respect to ground. The signals from the transducers measuring displacement in corresponding directions are then summed by means of a suitable analogue circuit to produce a vibration signal indicative of the absolute displacement of the shaft with respect to ground in each measured direction. In practice measurements are typically made on a horizontal and vertical axis each perpendicular to the shaft and in a direction parallel with the axis of the shaft. In this prior art arrangement is essential that the two transducers measuring displacement in the plane perpendicular to the shaft axis measure displacement in mutually perpendicular directions so that the obtained displacement signals can be displayed for example on a cathode ray oscilloscope as a function of time. Such a display produces a Lissajous figure which is the motion of the shaft axis and is termed the shaft orbit. The difficulty with this type of analogue information is that it is difficult to simultaneously display a meaningful representation of the displacement of the shaft in three directions simultaneously and it can only be done one bearing at a time for one harmonic at a time. The harmonics that can be analysed are also generally restricted to those having a frequency of one half, once, and twice shaft speed. In addition it is difficult and time consuming to perform any further harmonic analysis and calculations using the data since these must all be conducted by analogue means. In addition, the analogue signals can only be stored, for example, by recording on a tape recorder or some other suitable media.
Displacement measurement of rotating shafts in particular also requires correction for shaft imperfections or metal permeability changes that affect the transducer output. Accordingly, it is necessary to conduct a procedure known as a "slow roll" measurement which is the measurement of displacement at a slow shaft speed at which vibration effects can substantially be ignored and a calibration of the displacement measuring apparatus obtained relative to a known position on the shaft. This value is then subtracted from the absolute displacement data obtained as outlined above to give a true measure of absolute displacement. This subtraction requires a complex analogue filtering and off-setting process and can only be achieved for any one vibration harmonic at a time and further adjustment is required to perform the subtraction for each other harmonic of interest. The term harmonic as used herein is to be understood as including both integer and fractional harmonics. Since the subtraction is essentially by analogue means it is necessary to record the "slow roll" signal on a magnetic tape or other suitable device so that it can be reproduced for synchronous subtraction during vibration measurements at normal shaft rotating speeds. The storage of " slow roll" data is particularly important since some machines, such as turbine driven generators are stopped or slowed as infrequently as once every few years. "Slow-roll" data can only be obtained at these times and must be accurately stored for long periods. Hitherto, most storage has been by way of analogue tape recording which is clumsy to reproduce and not totally reliable.
Other disadvantages of the prior art analysing techniques include the restriction that because of the complexity of the analogue manipulation involved, vibration at each bearing in a machine for example, has to be measured and treated individually. This presents a serious limitation on the effectiveness with which a shaft can be analysed for balance quality in relation to harmonics of shaft speed frequency, alignment in relation to harmonics of shaft speed and twice shaft speed frequency, and looseness in relation to integer and fractional harmonics of three times shaft speed and greater frequency. The only effective balancing procedure is a process involving the placing of a number of trial weights at each balance plane and monitoring the difference of these weights at each bearing separately.
Because of the analogue nature of the vibration measurement data hitherto obtainable, it is not possible to accurately calculate various forces within the shaft so as to determine whether the vibration is caused for example by shaft imbalance, bearing misalignment, or other faults such as looseness and physical damage to the shaft.